The long-term goal of this project is to determine the role of the sorting proteins PACS-1 and PACS-2 in autosomal dominant polycystic kidney disease (ADPKD). This inherited disorder manifests in formation of numerous cysts in the kidney, culminating in excessive apoptosis and destruction of normal tissue. ADPKD is frequently caused by mutation of polycystin-2, a calcium-permeable ion channel that functions in multiple subcellular organelles. We recently identified the molecular trafficking machinery-PACS-1 and PACS-2- governing the stepwise movement of polycystin-2 between the endoplasmic reticulum (ER), Golgi and the cell surface. In addition, we discovered that PACS-1 and PACS-2 integrate protein trafficking with apoptosis and cell differentiation. We hypothesize that PACS-1 and PACS-2 are multifunctional sorting proteins that control the subcellular localization of polycystin-2 in the normal kidney, and that misregulation of PACS-1 and PACS-2 contributes to the cystogenesis and excess apoptosis observed in the polycystic kidney. We identified PACS-2 as the first COPI connector, and experiments in Aim 1 will determine how PACS-2 and COPI combine to localize polycystin-2 to the ER-the principle cellular reservoir for this ion channel. Also, we showed that PACS-1 is an AP-1 connector that localizes polycystin-2 to the trans-Golgi network (TGN). Studies in Aim 2 will determine how regulation of PACS-1 and PACS-2 sorting activity effects polycystin-2 calcium spikes in multiple subcellular compartments. Despite the causal relationship between polycystin-2 mutations and ADPKD, the steps leading from channel dysfunction to cystogenesis and disease are poorly understood. We recently found that PACS-1 expression is severely reduced in the ADPKD kidney, while PACS-2 expression is relatively little changed-a combination that favors apoptosis in cultured cells. Studies in Aim 3 will determine the cellular expression of PACS-1 and PACS-2 in the ADPKD kidney and will test whether loss of PACS-2 inhibits cystogenesis using a mouse model of polycystic kidney disease. Successful completion of our proposed studies will illuminate for the first time the multifunctional trafficking machinery- PACS-1 and PACS-2-that regulates the ability of polycystin-2 to conduct calcium currents in multiple organelles, and how misregulation of PACS-1 and PACS-2 expression contributes to the cystogenesis and excess apoptosis found in ADPKD.